Conflict
This module helps prevent accidental overwriting of content when multiple users edit simultaneously. It detects field-level conflicts by comparing original content, your edits, and others' edits to allow for merging changes.
Drupal 8
The module branch 8.x-2.x works out of the box for all kind of content entities - no configuration is needed.
Conflicts which will be auto merged:
-changes in translatable fields in non-edited content entity translations
-changes in fields to which the user does not have access
--fields with no edit access
--fields not part of the entity form display
-changes in entity metadata
--revision ID
--changed timestamp
There is also the ability for a conflict UI however currently no default conflict resolution is provided. This should be provided by - #2910910: Add default conflict resolution UI so please use this issue for your ideas and suggestions.
There is out of the box support for inline entity references, but for that the form alter hook should be called in a similar way.
Drupal 7
This module allows parallel editing of nodes with field-level conflict detection and a trick to prevent accidental overwriting of fields when they are changed by another session while a node is being edited.
This behavior is useful for things like issue queues where multiple people might be following up to an issue simultaneously. The original problem that lead to this module is #1559578: D7 solution for when multiple users are updating an issue simultaneously.
It works by doing a 3-way comparison between the field data of the original node, the node as edited by us, and the node as edited by others.